K-AR AND OXYGEN ISOTOPIC CONSTRAINTS OF ILLITIC CLAYS ON THE TIMING AND EVOLUTION OF THE WOODLEIGH IMPACT STRUCTURE, CARNARVON BASIN, WESTERN AUSTRALIA

Woodleigh is a recently discovered impact structure with a diameter of 120 km, representing the third largest proven Phanerozoic impact structure known after Morocweng and Chicxulub. An impact of this magnitude may correlate with one of the major mass extinctions. However, previous attempts to determine the age of the Woodleigh impact event had been unsuccessful as a result of incomplete isotopic resetting of biotites and zircons from associated impact rocks as well as post-impact isotopic overprinting of feldspars. We performed mineralogic, K-Ar and oxygen isotopic studies of fine-grained authigenic illitic clay minerals (<2-mm) that are common constituents of impact-induced hydrothermal alteration products. Illite K-Ar ages indicate a Late Devonian impact age, which coincides with several other impact events and a major global extinction in the Late Devonian some 365 Ma ago. Other evidences for Late Devonian extraterrestrial impacts compiled from the literature include the strong iridium anomaly in the Canning Basin, Western Australia, and microtektites and elemental anomalies in South China. Given the large diameter of the Woodleigh impact structure and its relative proximity to iridium anomalies of Late Devonian age in eastern Gondwana basins, Woodleigh can be considered as a likely major contributor to this extinction. Woodleigh impact-related clay and carbonate minerals have significantly higher median d18O values from 15.5 to 18.4 ‰. The calculated fluid d18O value ranges from 8.4 to 13.4 ‰. This fluid oxygen isotope composition is highly enriched in 18O relative to seawater, meteoric water and most basinal brines and overlaps the fields of magmatic and metamorphic fluids. Extensive interaction at relatively low water/rock ratios with 18O-rich rock types is required to explain the large 18O shift if the water in the impact related hydrothermal system was largely of surface derivation. Alternatively the hydrothermal fluids may have interacted with or incorporated 18O-rich condensed meteoritic components that were injected into the crater floor.